GLASS FIBRES

Glass fiber is one of the versatile fibers and possesses many unique properties with divergent applications, which other fibres cannot have. Glass is a man-made fibre and can be manufactured either in filament form or in staple form.

glass fibre        

RAW MATERIALS OF GLASS FIBRE

The principal ingredient of glass fibre is silica or silicon dioxide. However, depending upon the end-use, other materials like lime, magnesia, alumina, soda, potash or boric oxide are used. Based on this, there are three types of glass fibre. Those are :
  1.  Alkali type Glass or Glass 'A' (A = Alkali)
  2. Electrical type Glass or Glass 'E' (E = Electrical)
  3. Chemical type Glass or Glass 'C' (C= Chemical) %3D


The approximate compositions of different type of glass, fibers are shown in  Chemical Compositions of Glass Fibres

                        Glass A       Glass E       Glass C
  1. Silica%         72              52-56 (%)     62-65
  2. Lime              10             16-25             6
  3. Magnesia      3                0.6                -
  4. Alumina         2              12-16            1
  5. Soda             13              0.1               11-15
  6. Potash           -                0.1               1.3
  7. Boric oxide    -               8-13                3-4     




The principal ingredient i.e., silicon dioxide is generally obtained from sands or sandstones. The second important ingredient is lime, which is used for a stabilizing influence. Magnesium oxide has similar effects. Alumina is usually added to improve the strength, durability, and resistance

 to weathering. The other ingredients are generally added to improve the end-use applications. Sometimes zinc oxide is used to improve acid durability. The addition of titanium oxide reduces the viscosity of the melt. Barium oxide improves weathering characteristics and also it increases the melting rate.

FIBRE FORMATION OF GLASS FIBRE
The fibre formation from glass generally consists of the following processes.
(a) Preparation of the glass marbles
(b) Melting and Extrusion of the glass
(c) Filament or staple fibre formation.


PREPARATION OF THE GLASS MARBLES
The raw materials as per are mixed in a mixer. The exact amount of feeding can be done by means of weighing hoppers, which will transport the predetermined quantity of the raw material to the mixer. The mixer simply mixes all the materials homogeneously and uniformly. After mixing, the chemicals are generally transferred to a melter. In the melter, all the chemicals are melted. The molten materials are then converted into marble form and solidified.
The diameter of the marbles is approximately 2 cm. Before further processing, the marbles are generally inspected to check the defects, which may interrupt subsequent processing. These marbles are then transferred to the spinning hopper.

MELTING AND EXTRUSION
The marbles are present in the hopper transferred to the spinning unit by means of transfer pipe. The marbles are then melted in an electrical furnace. The temperature of the furnace is in the range of 800°C or above depending upon the type of the glass to be produced. The molten material is extruded through small orifices for the thread formation.


FILAMENT OR STAPLE FIBRE FORMATION
Glass is a supercooled liquid and thus is totally amorphous. The fibre can be drawn and/or stapled without much difficulty. The fine stream of liquid extruded through the spinnerette is generally deformed or drawn and collected in a winding tube after proper size application. The speed of the take up is around 1000 meters/minute. The filament thus formed can be further twisted and plied for their end uses. For staple fibre formation, the molten glass streams are converted into a staple fibre below the spinnerette by means of high-pressure air.

The air blower pulls the glass streams into fibres. The fibres are then collected together over a revolving drum. From the drum, a thin veil of fibers is pulled out just like pulling of carded webs from the doffer. The strand of fibres can be converted in sliver form by means of a ring guide and collected on a winding speed. The sliver can further be drafted, twisted like other fibres.

STRUCTURE OF LASS FIBRE

The chemical structure of the fibre is shown. Glass fibres are formed from the complex mixture of silica’s, oxides of sodium, potassium, calcium, aluminum, magnesium and other salts in varying composition. Owing to this, the structure of the glass fibre is more complex. The cation of silicon and oxygen ion consists of a network structure, where the silicon cation is surrounded by four oxygen ions. These are arranged in a tetrahedron with a cache of the four oxygen atoms at a corner and identical with the corner of the adjoining tetrahedron. In this manner, a continuous network structure is formed with only silicon and oxygen. In spite of this, the structure of the glass is of an irregular network having holes or interstices in it.
The holes are filled with other captions like calcium, sodium, potassium, etc as shown in fig These cations have relatively large ionic radii with small charges in comparison with the radius value of 0.14 micron of oxygen and 0.04 of silicon. The properties of the glass fibre continuously change with temperature because of these cations although glass fibres do crystallize and do not have any characteristic transitions. The presence of more cations decreases the value of the Si:O ratio. 

In a pure silica, the ratio is 0.5. Further it can be reduced to 0.25 if an equal amount of cation is introduced in glass structure i.e., equal amounts of silica and other oxides. A hard glass generally approaches to high Si:O ratio of 0.5 and it forms a pure polymeric chain of silicon bridging oxygen. On the other hand, a soft glass will have a low Si:0 ratio with more amount of non-bridging oxygen. In general, a high Si:O ratio is generally viewed as a high degree of polymerization and exhibits high softening temperature with a low coefficient of thermal expansion.

PHYSICAL PROPERTIES OF GLASS FIBRE

The properties of the glass fibre change as per the chemical composition or Si:O ratio. The fibre, is extremely dense, having a density of 2.5 to 2.6. The density of glass C is slightly higher than that of glass E. The fibre has no affinity to water and so the moisture regain of the fibre is maximum 0.5% or lesser. The fibres are extremely strong. The strength depends upon both the composition and the method of production. Fibers with boric oxide or borosilicate type of glass fibres i.e., glass E is the strongest fibre.

The tenacity in the dry state varies from 6 to 10 g/d, which reduces to 5 to 8 g/d when wet. The breaking elongation is only 3 to 4% but is perfectly elastic up to their breaking point. Glass fibres are quite stiff, brittle, break on bending and so exhibit poor abrasion resistance. The fibre softens, melts and does not burn upon heating. The fibre can be used continuously up to 500°C. The fibre becomes slightly brash and embrittled when passed over metal surfaces in the temperature range of 450°C - 480°C. The melting point of the fibre is around 750°C. The electrical resistance of the fibre is very high. The fibre exhibits excellent electrical insulation properties.

CHEMICAL PROPERTIES OF GLASS FIBRE

The chemical properties of the glass fibre depend upon their composition. The fibres are chemically inert to oxidizing agents, biological agents, heat, and sunlight under normal conditions. Alkali containing fibres are less resistant to weathering, have lower insulation resistance and dielectric strength than glass 'E'. However, most of the mineral acids like hydrofluoric, hydrochloric, sulphuric and phosphoric acids attack glass fiber. Also, hot solutions of weak bases and cold solutions of strong bases deteriorate the fibre. The fibre exhibits excellent corrosion resistant behavior. It is difficult to dye glass fibre. 

APPLICATIONS OF GLASS FIBRE


High tensile strength, low moisture absorption, higher utilization temperature, non-compatibility, high heat conductivity, better electrical resistance, higher corrosion resistance, better drapability, all contribute to using of this fibre extensively in furnishing fabric and industrial fabrics. These fibres are used in electrical the industry as the insulating material, fiberglass reinforced plastics for trucks and car bodies, thermal insulating materials, tire cord, industrial filters, protective clothing, decorative materials like curtains, and draperies, protective clothing against radiation, defense equipment, shipbuilding, etc.


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